This invention deals with a method of manufacturing alkylhalosilanes. More particularly, this invention deals with the use of phosphorous, certain phosphorous compounds or compounds that decompose to certain phosphorous compounds as promoters in the direct process for producing alkylhalosilanes. The benefits to be derived by the use of this invention can be increased alkylhalosilane yields, selectivity of certain alkylhalosilanes over other, less preferred alkylhalosilanes and, overall higher utilization of the raw materials used in the reaction mixture. In certain cases, using this invention, all three of these benefits can be had. In certain cases, using this invention, both the enhanced selectivity and the increased yields of alkylhalosilanes can be realized and, in certain other cases, the benefits of much higher conversion of raw materials to useable products is realized.
The Direct Process for producing alkylhalosilanes is well-known and has been refined and modified in many ways since Rochow first set forth the manner in which one could obtain alkylhalosilanes by contacting alkylhalides with silicon at elevated temperatures. The process is used for producing virtually all commercial alkylhalosilanes in the world today and Rochow's process was a significant deviation from the much more dangerous Grignard reaction for producing such silanes. Rochow in U.S. Pat. No. 2,380,995, issued Aug. 7, 1945 showed passing a gaseous stream of methylchloride into a heated tube where it contacted powdered silicon at about 300.degree. C. Rochow obtained a mixture of silanes, most predominantly CH.sub.3 SiCl.sub.3 and (CH.sub.3).sub.2 SiCl.sub.2 at 52 and 14.5 weight percent respectively, to give a Me/Me.sub.2 ratio (defined infra) of 3.6. It should be noted that Rochow also showed the passing of gaseous methylchloride over a powdered 50/50 by weight silicon-copper alloy and disclosed the use of metallic catalysts other than copper such as nickel, tin, antimony, manganese, silver and titanium, although the amounts and the physical forms of such catalysts are not disclosed by Rochow. Many silanes are formed by the Direct Process, such as, for example, tetramethylsilane, monomethyltrichlorosilane, silicon tetrachloride, trichlorosilane (HSiCl.sub.3), methyldichlorosilane MeHSiCl.sub.2 , dimethylchlorosilane {(CH.sub.3).sub.2 HSiCl}, and trimethylchlorosilane {(CH.sub.3).sub.3 SiCl}. In modern manufacture, the largest volume silane manufactured is dimethyldichlorosilane as this silane constitutes the backbone of most high volume commercial silicone products after it has been hydrolyzed and condensed to the siloxane form. It is to the benefit of the manufacturer then to run the Direct Process to maximize the conversion of the raw materials to realize the highest yield of dimethyldichlorosilane. Thus, one of the principal objectives of the instant invention is to control the Direct Process to maximize the yield of dimethyldichlorosilane, i.e. to cause the process to be more selective in favor of dimethyldichlorosilane. A second objective of the instant invention is to maximize the overall yield from the raw materials. The more of the raw materials that are converted to silanes, the more economical is the process.
For purposes of this invention, the efficiency of converting raw materials is tracked by the amount of silicon metal charge that is converted to silane (% Si conversion). Those skilled in the art are particularly interested in the selectivity of the direct process reaction and for the purposes of this invention, this selectivity is indicated as the ratio of monomethyltrichlorosilane to dimethyldichlorosilane (Me/Me.sub.2) in the crude reaction mixture. Sometimes, this ratio is also referred to as M/M.sub.2 or T/D in the literature. An increase therefore, in the Me/Me.sub.2 ratio, indicates that there is a decrease in the production of the more preferred dimethyldichlorosilane; conversely, a decrease in the ratio indicates that there is an increase in the production of the more preferred dimethyldichlorosilane.
When one considers that several millions of pounds of silanes are produced annually and consumed by the silicones commercial effort, it can be observed why small increments in selectivity, and raw material conversion are important to the manufacturer of silanes.
For example, assuming a manufacturer of silanes produces ten million pounds annually of silanes. If the process can be controlled to increase the overall yield of (CH.sub.3).sub.2 SiCl.sub.2 by 2 or 3 percent, then the process becomes particularly attractive.